ABSTRACT ? Project 2 Compared to infants of other races, African American (AA) infants are more likely to be exposed to environmental toxicants, to be born preterm, and to reside in poverty, whereas they are less likely to be breastfed. These disparities translate into higher risks for neurocognitive and social-emotional developmental delays. The microbiome ? which influences the brain through hormonal, and immunologic pathways ? is receiving growing attention as an important contributor to neurodevelopment. The gut microbiome is established at birth from maternal flora and varies according to perinatal factors such as mode of delivery (vaginal vs. Cesarean), gestational age and size, type of feeding (breast vs. bottle), and antibiotic exposure. Thereafter, diet, illness, vaccination, and antibiotic exposures further influence microbiome development. Thus, prenatal and postnatal exposures that affect the microbiome-gut-brain axis in the critical first years of life may significantly impact the developing brain. Although the microbiome resides at the interface between environmental exposures and the human host, the impact of environmental exposures on the microbiome remain largely unexplored. The proposed research will investigate whether the composition of the infant gut microbiome associates with exposure to prenatal and postnatal environmental toxicants and contributes to adverse neurocognitive and social-emotional outcomes for AA infants over the first 18- months of life. To accomplish this, we will leverage data from an on-going longitudinal study of preterm birth in AA women (R01 NR014800), that is enrolling a socioeconomically diverse cohort of ? 800 pregnant AA women and following them at 8-14 and 24-30 weeks? gestation through delivery. This proposed study of postnatal infant development will recruit mother-infant pairs post-delivery from the mothers enrolled in the existing Parent Study and Project 1 (aimed at characterizing pre- and postnatal exposures to environmental toxicants) in order to evaluate for the birthed AA infants, from 1-week through 18-months of age: (1) associations between prenatal and postnatal exposures to environmental toxicants and the composition of the infant gut microbiome; (2) infant neurodevelopmental outcomes associated with prenatal and postnatal exposures to environmental toxicants; and (3) the potential moderating influences of the microbiome and maternal caregiving on the relationship between postnatal toxicant exposures and infant neurocognitive and social-emotional development. Atlanta is home to AA women of broad socioeconomic status, providing a diverse sample of AA families, which allows for sufficient variation in the environmental factors under study to distinguish their independent and interactive effects on the microbiome and infant neurocognitive and socio- emotional development. Because many factors that influence the microbiome are modifiable, the knowledge gained by achieving our Aims holds potential for promoting the health of the next generation of AA families.